Field
The present invention relates to an organic electroluminescent device.
Related Art
An organic electroluminescent device (which may be hereinafter referred to as an organic EL device) is generally constituted by an anode, a cathode, and at least one layer of an organic thin film layer that intervenes between the anode and the cathode. On application of a voltage to the electrodes, electrons and holes are injected to the light emitting region from the cathode and the anode, respectively, and the electrons and the holes thus injected are recombined to form an excited state in the light emitting region. The device emits light on returning the excited state to the ground state.
An organic EL device provides various colors for the emitted light by using various light emitting materials in the emitting layer, and accordingly is being actively studied for practical applications, such as a display device. In particular, light emitting materials for the three primary colors, i.e., red, green and blue, are most actively developed, and earnest studies therefor are being made for enhancing the characteristics thereof.
One of the largest problems in an organic EL device is the achievement of both the high luminous efficiency and the low driving voltage. One of the known measures for providing a light emitting device having a high efficiency is to provide an emitting layer by doping several percents of a doping material to a host material. The host material is required to have a high carrier mobility, uniform film forming property and the like, and the doping material is required to have a high fluorescent quantum yield, uniform dispersibility and the like.
As these materials for the emitting layer, benzofluorene compounds are described, for example, in Patent Literatures 1 to 5.    Patent Literature 1: WO 07/148,660    Patent Literature 2: WO 08/062,636    Patent Literature 3: US-A-2007-0215889    Patent Literature 4: JP-A-2005-290000    Patent Literature 5: WO 2011/021520
However, the present inventors have found that the improvement in prolongation of the service life time is still insufficient even when the benzofluorene compounds described in Patent Literatures 1 to 5 are used, and further improvements are being demanded.